Running FreeNAS-11.1-U4 , It was working normal but yesterday i delete everything and wanted to reinstall  plex plugins and got this error I can install other plugins normally.. 
Code:
Request Method:    POST
Request URL:    http://10.0.1.49/plugins/plugin/install/8b2465b075e748638542562fa6f74a870141b2baf542a3715822c428fb5d7823/
Software Version:    FreeNAS-11.1-U4 (89e3d93bc)
Exception Type:    MiddlewareError
Exception Value:   
[MiddlewareError: Failed to download http://download.freenas.org/plugins/9/x64: HTTP Error 404: Not Found]
Exception Location:    ./freenasUI/plugins/plugin.py in download, line 120
Server time:    Fri, 29 Mar 2019 16:28:17 +0100
Traceback
FreeNAS® © 2019 - iXsystems, Inc. - 11.1-U4
Mar 29 15:30:50 TKD-NAS uwsgi: [middleware.exceptions:36] [MiddlewareError: Failed to download http://download.freenas.org/plugins/9/x64: HTTP Error 404: Not Found]
Mar 29 16:27:39 TKD-NAS uwsgi: [freeadmin.navtree:562] Couldn't retrieve http://10.0.1.49/plugins/couchpotato/2/_s/treemenu: timed out
Mar 29 16:28:17 TKD-NAS uwsgi: [middleware.exceptions:36] [MiddlewareError: Failed to download http://download.freenas.org/plugins/9/x64: HTTP Error 404: Not Found]
Install plugin
Request Method:    POST
Request URL:    http://10.0.1.49/plugins/plugin/install/8b2465b075e748638542562fa6f74a870141b2baf542a3715822c428fb5d7823/
Software Version:    FreeNAS-11.1-U4 (89e3d93bc)
Exception Type:    MiddlewareError
Exception Value:   
[MiddlewareError: Failed to download http://download.freenas.org/plugins/9/x64: HTTP Error 404: Not Found]
Exception Location:    ./freenasUI/plugins/plugin.py in download, line 120
Server time:    Fri, 29 Mar 2019 16:28:17 +0100
Traceback
Environment:
Software Version: FreeNAS-11.1-U4 (89e3d93bc)
Request Method: POST
Request URL: http://10.0.1.49/plugins/plugin/install/8b2465b075e748638542562fa6f74a870141b2baf542a3715822c428fb5d7823/
Traceback:
File "./freenasUI/plugins/plugin.py" in download
  106.                 rv = self.__download(rpath, path)
File "./freenasUI/plugins/plugin.py" in __download
  127.         response = urllib.request.urlopen(url)
File "/usr/local/lib/python3.6/urllib/request.py" in urlopen
  223.     return opener.open(url, data, timeout)
File "/usr/local/lib/python3.6/urllib/request.py" in open
  532.             response = meth(req, response)
File "/usr/local/lib/python3.6/urllib/request.py" in http_response
  642.                 'http', request, response, code, msg, hdrs)
File "/usr/local/lib/python3.6/urllib/request.py" in error
  570.             return self._call_chain(*args)
File "/usr/local/lib/python3.6/urllib/request.py" in _call_chain
  504.             result = func(*args)
File "/usr/local/lib/python3.6/urllib/request.py" in http_error_default
  650.         raise HTTPError(req.full_url, code, msg, hdrs, fp)
File "/usr/local/lib/python3.6/site-packages/django/core/handlers/exception.py" in inner
  42.             response = get_response(request)
File "/usr/local/lib/python3.6/site-packages/django/core/handlers/base.py" in _legacy_get_response
  249.             response = self._get_response(request)
File "/usr/local/lib/python3.6/site-packages/django/core/handlers/base.py" in _get_response
  178.             response = middleware_method(request, callback, callback_args, callback_kwargs)
File "./freenasUI/freeadmin/middleware.py" in process_view
  162.         return login_required(view_func)(request, *view_args, **view_kwargs)
File "/usr/local/lib/python3.6/site-packages/django/contrib/auth/decorators.py" in _wrapped_view
  23.                 return view_func(request, *args, **kwargs)
File "./freenasUI/plugins/views.py" in install_available
  298.         if not plugin.download("/var/tmp/firmware/pbifile.pbi"):
File "./freenasUI/plugins/plugin.py" in download
  120.                         'error': e,
Exception Type: MiddlewareError at /plugins/plugin/install/8b2465b075e748638542562fa6f74a870141b2baf542a3715822c428fb5d7823/
Exception Value: [MiddlewareError: Failed to download http://download.freenas.org/plugins/9/x64: HTTP Error 404: Not Found]
 
Request information
GET
No GET data
POST
Variable    Value
__form_id    'form_str'
FILES
No FILES data
COOKIES
Variable    Value
csrftoken    '********'
sessionid    'zlqbo3offyqmgvgpmtsvky63wxoj796m'
fntreeSaveStateCookie    'root'
META
Variable    Value